Reworkability method for wirebond chips using high performance capacitor

ABSTRACT

A device and method for enabling the reworkability of an integrated circuit comprising a wirebond chip having a bottom surface and a carrier substrate having a first surface and a second surface. The first surface and second surface of the carrier substrate are electrically connected through a series of vias. A bonding agent is used to mechanically attach the wirebond chip to the carrier substrate in addition to wirebonds for electrically connecting the wirebond chip to the substrate. The substrate is attached to a multi-chip module (MCM) by ball grid array (BGA) or controlled collapse chip connection (C4) attaching process.

This application is a divisional of U.S. patent application Ser. No.08/879,718, filed on Jun. 20, 1997, which has been allowed.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuits and, moreparticularly, to an apparatus and method for enabling the reworkabilityof an integrated circuit

BACKGROUND OF THE INVENTION

Circuit boards with multiple Very Large Scale Integrated (VLSI) circuitchips are called Multi-Chip Modules (MCM) The use of VLSI circuitspresents interface problems relating to the interconnection of theintegrated circuits to other circuits and the placement of theintegrated circuits on a ceramic circuit board (MCM-C) As VLSItechnology has advanced, the density of circuits on a single VLSI chiphas increased and the necessary interconnection for VLSI chips hasbecome increasingly difficult to achieve in a limited space.

In a typical configuration, semiconductor chips are mounted in cavitieson multilayer circuit boards or substrates and the substratesaccommodate intercircuit connections through tiny vertical holes or viasbetween the layers. In the case of wirebond chips, the chips areconnected to the vias using bonding wires which are welded to theinterconnection pads on the chip and the pads connected to the vias onthe substrate. The vias are filled with a conductive material, such asmolybdenum paste, which creates a connection to the VLSI circuit.

Reworkability is an issue for a wirebond chip which is attached to asubstrate. This is generally not an issue for Single Chip Modules(SCMs), where the chip carrier can be thrown away (with the chip) afterburn-in and test. For MCMs where only wirebond chips are used, however,reworkability is mandatory to prevent loss of the entire module, even ifbare die burn-in has preceded chip attachment.

FIG. 1 shows a typical application using a wirebond chip. In FIG. 1,wirebond chip 10 is mechanically attached to substrate 12 by bondingagent 14. Wirebond chip 10 is electrically connected to substrate 12 bybonding wire 16 at via 18. This prior art application has thedisadvantage that removal of wirebond chip 10 from substrate 12 resultsin loss of the module due to the nature of the removal process, as wellas the destruction of the wirebond chip, preventing defect analysis anddiagnostics of the chip.

Reworks in high speed MCMs are driven primarily due to speed imbalancesamong the individual chips on the MCM. This problem is exacerbated withComplementary Metal Oxide Semiconductor (CMOS) technology, where speedsorts of wafer level burn-in carriers are accurate to only within10-15%. So an MCM designed to run at 100 MHz may not function properlywith a microprocessor chip sorted at 90±10 MHz. This has been borne outby recent experiences with MCMs.

In addition to the reworkability issue, high performancemicros/Application Specific Integrated Circuit (ASIC) chips require alarge amount of decoupling. capacitance (1 to 3 μF). Because a wirebondchip image is significantly larger than an equivalentcontrolled-collapsed-chip-connection (C4) chip image on an MCM, onmodule discrete decoupling capacitors do not work as well as capacitorsthat lie directly beneath the chip. U.S. Pat. No. 5,095,402 issued toHernandez et al. illustrates a decoupling capacitor placed within anintegrated circuit package. As illustrated in FIG. 2, wirebond chip 20is attached to an IC carrier 22. Bonding wire 26 electrically connectswirebond chip 20 with pads 28 of carrier 22. A decoupling capacitor 24is attached to wirebond chip 20. This prior art also exhibits thedrawback that wirebond chip 20 cannot be removed from carrier 22 withoutdestroying the carrier.

FIG. 3 shows a typical MCM which has nine sites 32 for mounting dice 34,36, etc. If, for example, die 34 is defective due to an improperwirebond, solder joint, or speed intolerance, the entire MCM 30 must bescrapped because conventional mounting methods of dice 34, 36, etc. donot provide for non-destructive removal of the defective die

FIG. 4 illustrates a conventional method of mounting electroniccomponents 40 to a substrate 42. This conventional method uses solderballs 44 to attach component 40 to substrate 42. This method has adrawback, however, in that decoupling capacitors (not shown) must beattached to substrate 42 at locations remote from component 40. Inaddition, physical constraints limit the amount of decoupling availableto any given component. This results in insufficient decoupling of highfrequency noise resulting in inferior high speed performance of theassembled MCM.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide an apparatus andmethod for enabling the reworkability of an integrated circuit. It isanother object of the present invention to enhance the operation of anintegrated circuit. It is a further object of the present invention toprovide a carrier substrate to attach to a wirebond chip which allowsfor reworkability of an integrated circuit attached to a multi-chipmodule. It is another object of the present invention to provide areworkable multi-chip module with a decoupling capacitor integral witheach wirebond chip. Still another object of the present invention is toprovide a wirebond chip attached to a substrate which is attached to amulti-chip module using ball grid array (BGA) or controlled collapsechip connection (C4) attachment methods. It is another object of thepresent invention to provide a burn-in test vehicle capable of operatingat speeds of up to 800 MHz. Finally, it is another object of the presentinvention to allow non-destructive diagnostics of a chip wire bonded toa carrier.

SUMMARY OF THE INVENTION

An interposer is manufactured incorporating an integral capacitancelayer. The top surface of the interposer has wirebond pads to accept awirebond chip. The wirebond chip is die attached and then bonded to thepads on the top surface metallurgy (TSM). The wirebond pads on the TSMpads are connected to the bottom surface metallurgy (BSM) by thru vias.The BSM is C4 pads or BGA pads. Using this structure, a wirebond chipfrom a package can now be removed and replaced without the loss of theentire package. Previously, it was not possible to rework a wirebondchip, and thus wirebond chips were never placed on MCM's as onedefective or out-of-tolerance chip caused the entire module to bescrapped.

Using the integral capacitance layer interposer, when a wirebond chipfails, the interposer can be removed by either hot vacuum or in-situdevice removal. In hot vacuum, the module is placed in a box oven and atliquidous temperature and the interposer is lifted from the module bymeans of vacuum. In in-situ device removal, the interposer is grippedand lifted from the module at liquidous temperature through a beltfurnace using bimetallic disks to cause the lifting action. The modulesite is then dressed of residual solder by either copper block or shaveprocess and a replacement interposer is reattached to the module.

To solve the aforementioned disadvantages of conventional integratedcircuit mounting arrangements, the present invention relates to anapparatus and method for enabling the reworkability of an integratedcircuit. The apparatus comprises a wirebond chip and a carriersubstrate. The wirebond chip is attached to the top of the carriersubstrate and is electrically connected to the bottom surface of thecarrier substrate. The dielectric layer provides high decouplingcapacitance which is required to minimize power supply noise in highspeed processors.

The present invention also relates to an apparatus for enabling thereworkability and operation of an integrated circuit employing awirebond chip, a carrier substrate, and a dielectric layer attached tothe surface of the carrier substrate.

The present invention further relates to an apparatus for mounting awirebond chip to a module using a carrier substrate having structure forconnecting the carrier substrate to the wirebond chip, a dielectriclayer attached to the carrier substrate, and a device for attaching thebottom surface of the carrier substrate to the module.

The present invention relates still further to a method for mounting awirebond chip to a module comprising the steps of attaching a bottomsurface of a dielectric layer to a top surface of a carrier substrate,bonding a bottom surface of the wirebond chip to a top surface of thedielectric layer, coupling a plurality of electrical connections on thewirebond chip to a respective plurality of connections on the dielectricfilm, and coupling the bottom surface of the carrier substrate to themodule.

The present invention finally relates to a method for enabling thereworkability of the wirebond chip and the module comprising theadditional steps of decoupling the bottom surface of the carriersubstrate from the module and coupling a bottom surface of a furthercarrier substrate having a wirebond chip attached thereto to the module.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 is a partial side view of a prior art wirebond chip attachment;

FIG. 2 is a side view of a prior art integrated circuit with an integraldecoupling capacitor;

FIG. 3 is a plan view of a prior art multi-chip module;

FIG. 4 is a side view of a typical BGA attachment of devices to amulti-chip module;

FIG. 5 is a plan view of an exemplary embodiment of the presentinvention;

FIG. 6 is a sectional view of the exemplary embodiment of FIG. 5 throughsection 6—6;

FIG. 7 is a side view of an MCM of the present invention; and

FIG. 8 is a flow chart outlining a method of an exemplary embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 5, a top view of chip carrier 54 shows semiconductordie 52 which is wire bonded to chip carrier 54 with high dielectriclayer 56. Die 52 is mechanically bonded to chip carrier 54 by bondingmaterial (not shown), such as epoxy, for example. Die 52 is electricallyconnected to chip carrier 54 by wire leads 58. Wire leads 58 are bondedto surface pads 60 which are fabricated on top of the dielectric layer56. Wire leads 58 may be connected to a ground voltage plane (not shown)or to a power voltage plane (not shown), or may be a signal lead whichis not connected to any voltage plane. For those skilled in the art, itwill be apparent that any combination of the signal, ground, and powerconnections can be utilized depending on the specific performancerequirements of the semiconductor device.

Referring to FIG. 6, a partial sectional view of chip carrier 54 showsdie 52 attached to chip carrier 54 by wire leads 58. Chip carrier 54 hasfabricated on its surface a dielectric layer 56. The dielectric layer 56has metal mesh layers 62, 64 fabricated on both sides which act as thevoltage layers. For example, referring to FIGS. 5 and 6, chip connection66 is bonded to ground metal mesh layer 62 which is connected throughvia 68 and BGA 70 to a ground connection (not shown). Similarly, chipconnection 72 is bonded to voltage metal mesh layer 64 which isconnected through via 74 and BGA 76 to a voltage level used to power thechip. Typically, such voltage levels in microprocessor applications arebetween 2.5 and 5 volts. Finally, signal connection 78 from die 52 isconnected through via 80 to BGA 82. This signal connection 78 isisolated from any voltage or power mesh layers in the dielectric layer56.

Referring to FIG. 7, a side view of an MCM of the present invention isshown. In FIG. 7 carrier substrates 90, 92, 94, and 96 are shownattached to the multi-chip module 98 (MCM) utilizing the ball grid arrayassembly 100 (BGA). Each carrier substrate 90, 92, 94, and 96 has theintegral high dielectric thin film capacitor layer 56 as shown in FIG.6. Semiconductor dice, such as 52 shown in FIG. 5, are attached to MCM98 by a standard wirebond process used in the industry. After multiplecarrier substrates 90, 92, 94, and 96 are attached to MCM 98, MCM 98 istested for chip to chip connection at speed. MCM 98 may either be aproduction item or a test vehicle to test the carrier substrates 90, 92,94, and 96 at operational speeds or testing speeds. Typically, thesetesting speeds may range from 400-800 MHz or more. At this stage, due totiming issues associated with the industry, removal of a carriersubstrate, such as 90, 92, 94, and 96 from a multiple chip carryingsubstrate, due to failures or speed intolerances for example, is knownas “rework”.

A key advantage of the structure disclosed in this invention is thatindividual chip carriers 90, 92, 94, and 96 are attached to thesubstrate by an array of solder balls or BGA 100, for example. Otherattaching methods such as controlled collapse chip connection (C4) orlow melting point solder, for example, may also be used in place of BGA100. In this way, the defective carrier substrate 90, 92, 94, and 96 mayeasily be removed without destroying the underlying MCM 98. In addition,when MCM 98 is used as a testing vehicle, removal of carrier substrate90, 92, 94, and 96 which passed testing, is easily accomplished suchthat carrier substrate 90, 92, 94, and 96 may be used in further stagesof production, such as incorporation in a production module, forexample. This later procedure will enhance the throughput of MCMs byminimizing rework on production modules.

Carriers 90, 92, 94, and 96 attached to MCM 98 with BGA 100, can beremoved from an organic card or ceramic substrate, for example, byutilizing a process of melting the solder ball of BGA 100 and removingthe carrier substrate 90, 92, 94, and 96 during the time the balls aremolten. In FIG. 7, for example, if chip carrier 92 is found to bedefective, either due to a chip timing problems or a defectiveconnection between the chip die 52 and the MCM 98, the chip carrier 92can be removed from the MCM 98 without affecting other carriers such as90, 94 and 96. A new BGA chip carrier is then attached to MCM 98.

If the multiplicity of wirebond dice such as 52 were attached directlyto the MCM 98, individual removal of defective die, followed byreattachment using conventional wirebond methods, would not be possible.This invention allows high speed chips, which are designed for wirebondattachment, to be used on multi-chip carrying cards or ceramicsubstrates without the attendant drawbacks heretofore experienced.

FIG. 8 is a flow chart illustrating a method according to an exemplaryembodiment of the present invention. In Step 800, a chip carriersubstrate and high dielectric thin film capacitor is fabricated andtested. In Step 801, the wirebond chip is attached to the chipcarrier/thin film capacitor combination using conventional wirebondtechniques. In Step 802, the BGA is attached to the unoccupied surfaceof the carrier substrate. In Step 803, the carrier substrate containingthe wirebond chip, BGA, and thin film capacitor is tested. In Step 804,the carrier substrate which passed Step 803 is attached to a MCM. InStep 805, the MCM is tested at a predetermined is speed to identifydefective or out of tolerance circuits. In Step 806, if necessary, thedefective carrier substrates are removed from the MCM. In Step 807, anew chip carrier is attached to the MCM to replace the substrate removedin Step 806. Finally, in Step 808, the completely functional MCM iscompleted and passed for final installation or shipment. Thus, accordingto this exemplary method, a defective carrier substrate containing awirebond chip may be easily removed from the MCM without destroying theMCM thereby resulting in significant cost savings over conventionalmethods.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A method for mounting a wirebond chip to a modulecomprising the steps of: (a) attaching a bottom surface of a capacitivedielectric layer to a top surface of a carrier substrate, (b) bonding abottom surface of the wirebond chip to a top surface of the capacitivedielectric layer, (c) coupling a plurality of electrical connections onthe wirebond chip to a respective plurality of connections on thecapacitive dielectric layer, and (d) coupling the bottom surface of thecarrier substrate to the module, wherein the wirebond chip and themodule may be reworked.
 2. The method of claim 1, further comprising thesteps of: (e) decoupling the bottom surface of the carrier substratefrom the module, and (f) coupling a bottom surface of a further carriersubstrate having a wirebond chip attached thereto to the module.
 3. Amethod for mounting a wirebond chip according to claim 1, there in step(d) comprises the steps of: (d1) providing one of a ball grid array(BGA) and a controlled collapse chip connection (C4) between the bottomsurface of the carrier substrate and a surface of the module, (d2)coupling the bottom surface of the carrier substrate to the surface ofthe module with said one of said BGA and said C4.
 4. A method formounting a wirebond chip to a module comprising the steps of: (a)attaching a bottom surface of a capacitive dielectric layer to a topsurface of a carrier substrate, (b) bonding a bottom surface of thewirebond chip directly to a top surface of the capacitive dielectriclayer, (c) coupling a plurality of electrical connections on thewirebond chip to a respective plurality of connections on the capacitivedielectric layer, and (d) coupling the bottom surface of the carriersubstrate to the module, wherein the wirebond chip and the module may bereworked.
 5. The method of claim 4, further comprising the steps of: (e)decoupling the bottom surface of the carrier substrate from the module,and (f) coupling a bottom surface of a further carrier substrate havinga wirebond chip attached thereto to the module.